In recent years, in a field of dampers (shock absorbers) used in a vehicle suspension system, various types of variable damping force dampers that can variably control the damping force in steps or continuously have been developed. When conducting the damping force control in an automobile equipped with a variable damping force damper, a sprung mass velocity of each wheel is used as an important control parameter. Because it is substantially impossible to directly detect the sprung mass velocity in a traveling automobile, it is common to detect a vertical acceleration of the vehicle body for each wheel by means of an acceleration sensor and estimate the sprung mass velocity from the detected acceleration according to an oscillation model (observer) (see U.S. Pat. No. 5,987,367, for example).
However, the sprung mass velocity estimated by the above method may not be necessarily highly accurate because there may be variation in the acceleration values detected by the acceleration sensors and/or the computation precision according to the oscillation model may not be sufficiently high. For example, when the estimated sprung mass velocities for the four wheels are represented by heights of points (VBfl, VBfr, VBrl and VBrr) at corresponding positions as shown in FIG. 9, the four points VBfl-VBrr should be in a same flat plane because they result from a dynamic motion of the vehicle body that is considered a rigid member, but they often are not because of the estimation inaccuracy and the plane passing the four points is distorted. Thus, if the damping force control were conducted using such inaccurately estimated sprung mass velocities as control parameters, an unnecessary bending force may be applied on the vehicle body and/or riding comfort or driving stability can be deteriorated.